In recent years, there has been an increasing tendency of adopting liquid crystal panels as display devices on vehicles. In these liquid crystal panels, there is a configuration in which a cold-cathode tube is used as a back light which works as a light source. In general, the cold-cathode tube has a predetermined working temperature range, which is set to obtain a desired luminance. In particular, the cold-cathode tube has characteristics that the desired luminance cannot be obtained immediately after it is switched on, because the temperature in the interior of the cold-cathode tube is lower than the working temperature range set. Then, there have been proposed techniques in which the liquid crystal panel is controlled to be heated to the desired temperature by the use of a heater or the like (for example, refer to Patent Document 1).
FIG. 1 is a functional block diagram showing a schematic configuration of an LCD heater apparatus 110 for controlling a liquid crystal panel to stay within a desired temperature range, and the block diagram summarizes briefly and illustrates the technique disclosed in Patent Document 1 above. The LCD heater apparatus 110 includes a CPU 20, a heater control unit 140 and an LCD heater unit 70.
The CPU 20 includes a first port P10 and a second port P20 which are digital output ports for outputting a high or low signal, and an analog input port AN. Here, the second port P20 is not used. A thermistor 74 is connected to the analog input port AN via a sensor part 45 of the heater control unit 140.
The LCD heater unit 70 includes a heater 72 which is made up of a resistor, and the thermistor 74 which functions as a temperature sensor and is disposed in the vicinity of an LCD panel 80.
The heater control unit 140 includes the sensor part 45, a downstream-side controller 50, and a diode 41 for supplying power from a constant voltage power supply Vcc2 towards the heater 72. Specifically, an anode of the diode 41 is connected to the constant voltage power supply Vcc2, and a cathode of the diode 41 is connected to the heater 72. In addition, in a configuration in which the LCD heater apparatus 110 is installed on a vehicle, the constant voltage power supply Vcc2 is made to be interlocked with the on and off states of, for example, an ignition switch to supply power to the heater 72.
In the sensor part 45, a sensor resistor 43 and a capacitor 42 are connected in series between a constant voltage power supply Vcc1 and a ground potential. In addition, the thermistor 74 of the LCD heater unit 70 is connected between a connecting point T1 between the sensor resistor 43 and the capacitor 42 and the ground potential. In addition, the connecting point T1 is connected to the analog input port AN of the CPU 20. By this configuration, a change in resistance value of the thermistor 74 due to a change in temperature is inputted into the analog input port AN as a voltage change (partial pressures of the sensor resistor 43 and the thermistor 74), and the CPU 20 calculates a temperature in the thermistor 74 based on the input so made.
The downstream-side controller 50 is disposed on a downstream side of the heater 72 and includes a first switching device (a first switching unit) Tr1, a first resistor 52, and a second resistor 53. Specifically, the first switching device Tr1 is an n-channel FET (Field Effect Transistor), and a drain terminal D is connected to the heater 72, while a source terminal is connected to the ground potential. In addition, a gate terminal G is connected to the first port P10 via the first resistor 52. Resistance values of the first resistor 52 and the second resistor 53 are set in accordance with a voltage (a high voltage) that is applied to the gate terminal G and the switching characteristics of the first switching device Tr1. Additionally, when the first port P10 becomes high, the gate terminal G of the first switching device Tr1 becomes high, and the first switching device Tr1 is switched on. As this occurs, power is supplied from the constant voltage power supply Vcc2 to the heater 72, whereby the heater 72 heats the LCD panel 80. By this configuration, the CPU 20 on/off controls the downstream-side controller 50 (the first switching device Tr1) so as to control the supply of power to the heater 72 based on the temperature in the thermistor 74 sensed, whereby the LCD panel 80 is made to stay within the desired temperature range.
In addition, with a view to enhancing the display characteristics and enhancing the life of the LCD panel, there have been proposed techniques in which the temperature control is implemented in detail so that a back light of an LCD panel is turned off when the temperature of the back panel is low (for example, refer to Patent Document 2).